The objective is to understand how spermatogenesis and the rate of sperm production are regulated or modulated. Degeneration of germ cells is a common phenomenon of spermatogenesis which reduces theoretical values of daily sperm production in both animals and men. As high as 75% loss of theoretical values occurs during spermatocytogenesis. However, little is known about its mechanism, causes, relationship to somatic cell populations, or its prevention. To evaluate the roles of degeneration and number of stem cells on daily sperm production, both quantitative and autoradiographic studies will be conducted using light microscopy. We have shown that changes in numbers of A spermatagonia are largely responsible for seasonal variation in daily sperm production in stallions. This study will determine if increased number of A spermatogonia can be explained by increased number of stem cells or by less degeneration among subtypes of spermatogonia. Evaluation of stem cell number and degeneration of spermatogonia in different seasons, in which differences in daily sperm production are known to occur naturally, will reveal which aspects in spermatocytogenesis are flexible and possible amenable to intervention. The long-term labelling experiment should aid in evaluating the "reserve" stem cell concept as these cells should not be labelled if they are dormant in testes with active spermatogenesis. Interrelationships between spermatogonia and the Sertoli cell or Leydig cell population will be evaluated. Through autoradiographic and quantitative studies, the role of mitosis on number of Sertoli cells and Leydig cells will be evaluated. The temporal relationships between daily sperm production, stem cell number, germ cell degeneration, size of populations of Sertoli cells and Leydig cells, and hormone concentrations will be evaluated. Electron microscopy will be used to support light microscopy. The seasonally regressed stallion testis appears to be useful in evaluating the interrelationships of changes in the aging human testis. Both exhibit reduced sperm production rate, reduced numbers of spermatogonia, and reduced numbers of Sertoli cells and Leydig cells. The gametic function of the aging human testis is becoming more important as more people live longer. Knowledge of how sperm production rate may be regulated also is useful to human family planning programs involving oligospermic men and contraception strategies.